Back in the 1990s, GM wasn’t making too many waves in the diesel truck market. The 6.2L and 6.5L engines had been around for sometime, but they were no match for the release of the Cummins 6BT in the Dodge truck in 1989 and the Ford Powerstroke in 1994.

GM never really had a strong reputation for diesel design anyway.

All we remember is the GM diesel engines were a clacking bucket of bolts that were plagued by many issues from the start of the late 1970s.

GM’s goal at that time of the fuel crunch was to introduce an engine with great reliability and good fuel mileage. They weren’t interested in making power, just dependability.

By the mid-1990s, GM was definitely behind in both areas and had to find a way to compete in a growing diesel market. But by 1999, GM didn’t even offer a diesel option in any of its mid-size trucks.

Since 1976, GM and Isuzu have worked together as a joint venture on several projects. In 1996, the two automakers started working on a new mid-size diesel engine, and after two years of design, the Duramax was formed; production began in 2000.

The engine design was a 90° V8, iron block, with aluminum heads, 32 valves, common rail injection. It was ­turbocharged and intercooled and produced 300 hp and 520 ft.-lbs. of torque.

The engine was offered in GM mid-size diesel trucks at the start of 2001, and it brought GM back as a major player to the mid-size truck market by offering dependability, fuel mileage and quiet operation with plenty of power.

The question that I often hear about the Duramax is “What kind of problems do you see with these engines?”

I have to admit that these engines have a great reputation. While some problems have plagued a few of the Duramax engines, there hasn’t been anything as detrimental as what other manufacturers have had to deal with.

There are always going to be issues with anything that has to do with internal combustion. The best way to examine the Duramax would be to break down the engines by their generation and evaluate issues of each.

There are five ­generations of the Duramax engine. The best way to identify the generation is by the year, model and the RPO (regular production option) code.

The first generation is known as the LB7. This was manufactured from 2001 to 2004, with the eighth digit of the VIN designated with the number 1.

With the LB7 being the first design, the integrity of the engine was great; the biggest problem was the fuel system.

A very common problem was injector failure.

The engine incorporated the Bosch common rail fuel system, which was composed of the high-pressure fuel pump, high-pressure fuel rail, hard lines, injectors and electronic control module.

For some reason, the Bosch injectors could not sustain life in the Duramax.

The injectors would fail in three different ways  all related to the injector’s body becoming cracked.

One symptom of a cracked injector is excessive white smoke coming from the tailpipe, which is generally most noticeable at idle, especially while sitting in traffic.

Fuel is leaking into the combustion chamber, which cannot be controlled by the engine’s ECU. Fuel is entering the combustion chamber at the wrong time, causing the white smoke, which is an unburned fuel condition.

This can be seen with a scan tool and is known as an injector’s balance rate. The balance rates of an injector are adjustments of fuel to the injector made by the ECU.

The adjustments are made by the ECU from fluctuations of the crankshaft detected by the crankshaft position sensor.

The balance rates are given by the value being a plus or a minus to the volume of fuel per cylinder.

If there is too much fuel for a ­particular cylinder, the balance rates for that cylinder would be a minus. The ECU would be trying to take fuel away to correct the imbalance condition.

If there were leaking injectors in this fashion where there is white smoke, the balance rate for that cylinder would be a minus.

Another symptom of cracked injector failure would be fuel dilution in the engine oil. This would come from the injector’s body being cracked externally, causing fuel to leak into the crankcase.

You have to remember that the injectors of the LB7 were under the valve cover. The hard lines come from the high-pressure fuel rail and went through the valve cover.

So if the injectors were leaking externally, fuel dilution could happen fairly quickly and go unnoticed because the engine would operate fine.

There have been some injectors leaking externally so badly that the crankcase had filled with so much diesel that it was coming from the rear main seal.

The particular truck I was working on with this condition was dripping diesel fuel from the rear main seal without the engine even running in the parking lot.

The last form of injector failure was a hard start condition when the engine was hot.

When the engine was cold, the vehicle would start fine and drive normally without any noticeable problems until the owner decided to stop somewhere like the store to get some fuel.

When the owner would try to start the vehicle, the engine would spin over but never fire.

The injector’s body was cracked on the return side, causing the fuel pressure that was entering the injector to be returned to the fuel tank.

The vehicle would literally have to sit for several hours and cool down before the engine would restart. The heat from the engine would cause the crack in the injector body to expand open.

That’s why the engine will start fine when cold and struggle to crank when warm.

With so many injector failures between 2001 and 2004, GM extended the injectors’ warranty from five years/100,000 miles to seven years/200,000 miles. This did not, however, remedy the problem. Bosch went through several designs before there seemed to be a cure.

The biggest problem came when customers had their injectors replaced under the seven-year/200,000-mile warranty.

Then several years later, after the truck was out of warranty, the injectors failed again.

This of course angered many customers because there was still a problem and now they were going to have to pay for it.

But, if the other injectors didn’t last very long, this would be an ongoing problem for the owner.

The injector replacement in the LB7 Duramax is labor intensive.

With the injectors being under the valve cover, a lot of the components of the top of the engine have to be removed in order to access them.

It’s always advised that if there are several injectors causing problems that it’s better to replace them all because of the amount of labor that it takes to get to the injector.

The average cost of an injector replacement on the LB7 is generally around $4,000 to $5,000. The replacement process takes between 10 to 12 hours of labor and the injectors cost around $350 each.

In the middle of 2004, GM released the second generation of the Duramax, with the RPO code of LLY, with the eighth digit of the VIN designated as number 2.

The LLY was in production from 2004.5 to 2006, and was made with 310 hp and 605 ft.-lbs. of torque.

There were several reasons for the change: the injectors changed design and were now on the outside of the valve covers, providing easier access, and the EPA was tightening down on emissions standards for diesel engines in order to reduce NOx gas.

The LLY incorporated the use of an EGR (exhaust gas recirculation) valve. When engine conditions would reach a certain criteria determined by the ECU, the EGR valve would open to let exhaust gas to be reintroduced into the intake manifold.

Oftentimes this would cause a buildup of soot in the intake system due to the exhaust gas displacing the oxygen, which would cause cooler combustion; the cooler combustion inside the cylinder formed soot.

In order to reintroduce exhaust gas into the intake of a diesel engine, the exhaust gas has to pass through what is known as an EGR cooler. A diesel engine exhaust temperatures can be much higher than gasoline, reaching as high as 1,200° F.

Before the exhaust gas reenters the intake at this temperature, it has to be cooled. The EGR cooler is more or less a small radiator that is a part of the engine’s cooling system, which as the hot exhaust gas passes through the cooler will cool the exhaust gas before reaching the intake manifold.

Over a period of time, the EGR coolers will fail, causing the engine coolant to enter the intake manifold. This will often result in loss of coolant with steam emitting from the tailpipe.

The LLY suffered from overheating. When Duramax incorporated the use of the EGR cooler, the cooling system of the engine was not upgraded.

When towing with the LLY up steep grades on a hot summer day, owners often noticed that the cooling system could not sustain the engine’s temperature and would overheat.

One of the other features of the LLY was the use of VNT (variable nozzle turbo). The VNT was where the turbocharger could change the speed of the turbo by altering exhaust pulses to the turbine wheel.

This created better spool time and more boost for the engine off idle and would also change spool at the turbo when the engine was at cruising speed for the use of less boost. This is more or less letting the turbo make boost when there is a demand.

The VNT was sometimes responsible for the overheating issues of the LLY because of the more restricted exhaust system. Some overheating issues did result, however, in head gasket failures, which could damage the entire engine.

In 2006.5 through 2007, the Duramax changed generation(third) again to the RPO code LBZ. Under the LBZ, the Bosch fuel engine management system also changed. This time, the fuel system used a new 32-bit controller along with seven hole injectors.

The fuel pressures increased from 23,000 psi to 26,000 psi. Fuel sprayed directly onto the glow plugs for faster starts. The glow plugs were also independently controlled from the use of a controller for more efficiency during cold starts.

The block was redesigned with more integrity along with the pistons and rods for the increase in horsepower to 360 and 650 ft.-lbs. of torque. The cooling system was upgraded with the use of a bigger radiator and fan along with a bigger EGR cooler.

Owners of an LLY can upgrade the cooling system of their vehicles by installing the radiator and fan along with the fan shroud from an LBZ. The Allison transmission also changed from 5 speed to a 6 speed. The additional gear in the transmission reduced cruising speed by 200 rpm.

From 2007 to 2010, the (fourth) generation of Duramax changed again to RPO code LMM. The eighth digit of the VIN is designated with the number 6.

The LMM makes 365 hp and 660 ft.-lbs. of torque. Because the emissions standards for diesel engines were changing for lower NOx gas, the LMM incorporated the use of a DPF (diesel particulate filter) in the exhaust system.

The DPF is a device that is located behind the catalytic converter that traps soot coming from the engine. The DPF is monitored by the engine’s ECU by the use of pressure sensors located in the exhaust system.

When the DPF becomes clogged with soot, the ECU will actuate the injectors on the exhaust stroke, which will dump raw fuel into the DPF.

The fuel ignites in the exhaust system, which burns away the soot from the filter in the DPF. This was an effective way to rid soot from the tailpipe, but causes more fuel consumption.

For 2011 to the present, the (fifth) generation of the Duramax is currently RPO code LML. Horsepower has been increased to 397 along with torque to 765 ft.-lbs. The LML incorporates the use of urea injection. Urea injection is an exhaust after treatment, which further reduces emissions.

Urea is injected downstream of the turbo, which becomes a catalyst for NOx gas. Also, the fuel injection has changed to accommodate the use of piezo injectors and injection pressures reaching 29,000 psi.

Piezo injectors incorporate piezo crystals that are used to create movement of the injector’s pintle, which is faster than the traditional electromagnet.

This, along with the higher injection pressure, further increases engine efficiency.

With more than 10 years of engine operation, the Duramax is a proven player in the mid-size diesel world. The engine design has always been on the same platform with only improvements to the integrity of the design as power increased and fuel injection changes as emissions reductions became greater.

There have been some unusual failures such as broken rods or pistons, but very few accounts.

The biggest problem was the injector flaws from the LB7. This was the most major failure that put a bad taste of Duramax into owners’ mouths.

Other than this, there are occasional mechanical failures that can often be associated with any engine design. Parts over time become tired, it’s just related to what the vehicle is used for and how much maintenance is performed.

Keep in mind that the repair costs of the Duramax aren’t cheap. The parts will be competitive to other brands of diesels, but the labor is not.

The labor associated with the Duramax, depending on what the repair is, can be intensive. Take for instance a blown headgasket repair, the labor for both headgaskets on a Duramax will be around 35 hours compared to a Ford 6.0L that takes around 22 hours.